L-menthol, which has the structure as shown in formula (I), is in wide application thanks to its unique cool and refreshing effect. Currently, its global consumption amount is about 40 thousand tons annually, playing an important role in the flavor industry. There are two ways for obtaining L-menthol: natural extraction and chemical synthesis. Currently, natural extraction is still the dominant way of production, accounting for about 80% of the market share. The rest of 20% or so is made through chemical synthesis, comprising racemized menthol splitting synthesis method and asymmetric synthesis method. Currently, due to being subject to the limit of chiral catalyst and transition metal catalyst, asymmetric synthesis method is applied by a few companies only. Although the method of synthesizing L-menthol from racemized menthol is simple, one problem of the method is that D-menthol (II) at amount equivalent to L-menthol is produced. However, currently, the market capacity for D-menthol is very less. Presently, the solution is generally like this: translating D-menthol into racemized menthol first, then splitting to obtain L-menthol, repeating such process until it is fully translated into L-menthol. Besides, the most common method for synthesizing racemized menthol is from thymol hydrogenation. However, based on the current technical methods, it's very difficult to control the reaction to produce racemized menthol only, high proportion of product has resulted in other three pairs of isomers. Therefore, in this method of synthesizing L-menthol, how to efficiently translate D-menthol and other three pairs of isomers into racemized menthol is the critical factor for obtaining L-menthol at high yield.

Introduced below are a few reported methods for translating D-menthol and other three pairs of isomers into racemized menthol:
DE 2314813A1 describes hydrogenation of thymol using a Co/Mn catalyst fixed bed at 170˜220° C. temperature and 20 MPa pressure. The embodiments of that patent are implemented at 180˜210° C. temperatures and over 20 MPa pressure, and a mixture of 8 kinds of menthol stereoisomers is produced, consisting of 56.5˜59% of racemized menthol and 10.6˜10.8% of racemized isomenthol. And 4˜5% of light constituents exist in the mixture, which are non-recyclable by-products.
Bayer Corporation of Germany describes in U.S. Pat. No. 5,756,864 that: in fixed bed reaction, Ni, Mn and alkaline earth metal hydroxide or oxide without carrier were used as a catalyst to isomerize D-menthol under the condition of 200˜250° C. temperature and 100˜300 bar hydrogen pressure to obtain racemized menthol of 59.8%, the catalyst used in the reaction remains highly active 6000 hours after the reaction.
Patent Application WO 2012/010695A1 describes the method that adopts γ-Al2O3 carrier-loaded Ru catalyst and rare earth catalyst containing alkali metal to translate menthol for rearrangement isomerization, the method can obtain D,L-menthol of 68% under the reaction condition of 90˜120° C. and 1˜1.2 Mpa, and the ratio of D,L-menthol/D,L-isomenthol is 6˜8.5.
Patent Publication CN 103143363 A invented Cu/Ni catalyst with aluminium oxide as a carrier, which can achieve menthol content of up to 62.8% after the diastereoisomer of the menthol is isomerized.